WO2002005255A1

WO2002005255A1 - Current driven electrooptical device, e.g. organic electroluminescent display, with complementary driving transistors to counteract threshold voltage variation

Info

Publication number: WO2002005255A1
Application number: PCT/GB2001/003100
Authority: WO
Inventors: Simon Tam
Original assignee: Seiko Epson Corporation
Priority date: 2000-07-07
Filing date: 2001-07-09
Publication date: 2002-01-17
Also published as: CN1221933C; TW200302444A; CN1658266A; TWI282080B; US6919868B2; EP1170719A1; US20020021293A1; CN1877680A; CN100481185C; EP1170719B1; ATE524804T1; TWI277056B; CN1388952A

Abstract

A driver circuit comprises a p-channel transistor and an n-channel transistor connected as a complementary pair of transistors to provide analog control of the drive current for a current driven element, preferably an organic electroluminescent element (OEL element). The transistors, being of opposite channel, compensate for any variation in threshold voltage ΔVT and therefore provide a drive current to the OEL element which is relatively independent of ΔVT. The complementary pair of transistors can be applied to either voltage driving or current driving pixel driver circuits.

Description

CURRENT DRIVEN ELECTROOPTICAL DEVICE, E.G. ORGANIC ELECTROLUMINESCENT DISPLAY, ITH COMPLEMENTRAY DRIVING TRANSISTORS TO COUNTERACT THERSHOLD VOLTAGE VARIATION

The present invention relates to a driver circuit. One particular application of such a driver circuit is for driving an organic electroluminescent element.

An organic electrolurninescent (OEL) element OEL elementcomprises a light emitting material layer sandwiched between an anode layer and a cathode layer.

Electrically, this element operates like a diode. Optically, it emits light when forward

biased and the intensity of the emission increases with the forward bias current. It is

possible to construct a display panel with a matrix of OEL elements fabricated on a transparent substrate and with at least one of the electrode layers being transparent. . It is

also possible to integrate the driving circuit on the same panel by using low temperature

poly silicon thin film transistor (TFT) technology.

In a basic analog driving scheme for an active matrix OEL display, a minimum of

two transistors are required per pixel. Such a driving scheme is illustrated in Figure 1.

Transistor Tj is provided to address the pixel and transistor T is provided to convert a data

voltage signal N_Data into current which drives the OEL element at a designated brightness.

The data signal is stored by a storage capacitor C_st_orag_e when the pixel is not addressed.

Although p-channel TFTs are shown in the figure, the same principle can also be applied for a circuit utilising n-channel TFTs.

There are problems associated with TFT analog circuits and OEL elements do not

act like perfect diodes. The light emitting material does, however, have relatively uniform

characteristics. Due to the nature of the TFT fabrication technique, spatial variation of the TFT characteristics exists over the extent of the display panel. One of the most important

considerations in a TFT analog circuit is the variation of threshold voltage, ΔN_T, from

device to device. The effect of such variation in an OEL display, exacerbated by the non

perfect diode behaviour, is the non-uniform pixel brightness over the display area of the

panel, which seriously affects the image quality. Therefore, a built-in circuit for

compensating a dispersion of transistor characteristics is required.

A circuit shown in figure 2 is proposed as one of built-in for compensating a

variation of transistor characteristics. In this circuit, transistor T is provided for addressing

the pixel. Transistor T operates as an analog current control to provide the driving current

to the OEL element. Transistor T 3 connects between the drain and g °ate of transistor T 2 and toggles transistor T to act either as a diode or in a saturation mode. Transistor T acts

as a switch in response to an applied waveform N_GP. Either Transistor T or transistor T

can be ON at any one time. Initially, at time ^ shown in the timing diagram of Figure 2,

transistors T 1 and T3 are OFF, and transistor T is ON. When transistor T is OFF,

transistors T and T₃ are ON, and a current I_DAT of known value is allowed to flow into the

OEL element, through transistor T . This is the programming stage because the threshold

voltage of transistor T₂ is measured with transistor T₃ turned ON which shorts the drain and

gate of transistor T Hence transistor T operates as a diode while the prograrnming

current is allowed to flow through transistors T and T and into the OEL element. The

detected threshold voltage of transistor T is stored by a capacitor C connected between the

g °ate and source terminals of transistor T 2 when transistors T 3 and T 1 are switched OFF.

Transistor T is then turned ON by driving waveform V_GP and the current through the OEL element is now provided by supply N_DD. If the slope of the output characteristics for

transistor T were flat, the reproduced current would be the same as the programmed

current for any threshold voltage of T detected and stored in capacitor C However, by

turning ON transistor T , the drain-source voltage of transistor T is pulled up, so a flat

output characteristic will maintain the reproduced current at the same level as the

programmed current. Note that ΔN_T2 shown in figure 2 is imaginary, not real. It has been

used solely to represent the threshold voltage of transistor T .

A constant current is provided, in theory, during a subsequent active programming

stage, which is signified by the time interval ^ to t in the timing diagram shown in figure

2. The reproduction stage starts at time t •

The circuit of Figure 2 does provide an improvement over the circuit shown in

Figure 1 but variations in the threshold value of the control transistor are not fully

compensated and variations in image brightness over the display area of the panel remain.

The present invention seeks to provide an improved driver circuit. In its application

to OEL elements the present invention seeks to provide an improved pixel driver circuit in

which variations in the threshold voltages of the pixel driver transistor can be further

compensated, thereby providing a more uniform pixel brightness over the display area of

the panel and, therefore, improved image quality.

According to a first aspect of the present invention there is provided a driver circuit

for a current driven element, the circuit comprising an n-channel transistor and a

complementary p-channel transistor connected so as to operatively control, in combination,

the current supplied to the current driven element.

Beneficially, the current driven element is an electroluminescent element. Preferably, the driver circuit also comprises respective storage capacitors for the n-

channel and p-channel transistors and respective switching means connected so as to

establish when operative respective paths to the n-channel and p-channel transistors for

respective data voltage pulses.

Advantageously, the driver circuit may also comprise respective storage capacitors

for storing a respective operating voltage of the n-channel and the p-channel transistors

during a programming stage, a first switching means connected so as to establish when

operative a first current path from a source of current data signals through the n-channel and

p-channel transistors and the current driven element during the programming stage, and a

second switching means connected to establish when operative a second current path

through the n-channel and p-channel transistors and the current driven element during a reproduction stage.

In a further embodiment, the first switching means and the source of current data

signals are connected so as to provide when operative a current source for the current

driven element.

In an alternative embodiment, the first switching means the source of current data

signals are connected so as to provide when operative a current sink for the current driven

element. "

According to a second aspect of the present invention there is also provided a

method of controlling the supply current to a current driven element comprising providing

an n-channel transistor and a p-channel transistor connected so as to operatively control, in

combination, the supply current to the current driven element.

Preferably, the method further comprises providing respective storage capacitors for

the n-channel and p-channel transistors and respective switching means connected so as to establish when operative respective paths to the n-channel and p-channel transistors for

respective data voltage pulses thereby to establish, when operative, a voltage driver circuit

for the current driven element.

Advantageously, the method may comprise providing a prograrmning stage during

which the n-channel and p-channel transistors are operated in a first mode and wherein a

current path from a source of current data signals is established through the n-channel and

the p-channel transistors and the current driven element and wherein a respective operating

voltage of the n-channel transistor and the p-channel transistor is stored in respective storage

capacitors, and a reproduction stage wherein a second mode and a second current path is

established through the n-channel transistor and the p-channel transistor and the current

driven element.

Beneficially, the present invention provides a method of controlling the supply

current to an electroluminescent display comprising the method of the invention as described

above wherein the current driven element is an electroluminescent element.

According to a third aspect of the present invention, there is also provided an

organic electroluminescent display device comprising a driver circuit as claimed in any one

of claims 1 to 12.

The present invention will now be described by way of further example only and

with reference to the accompanying drawings in which: -

Fig. 1 shows a conventional OEL element pixel driver circuit using two transistors;

Fig. 2 shows a known current programmed OEL element driver circuit with

threshold voltage compensation; Fig. 3 illustrates the concept of a driver circuit including a complementary pair of

driver transistors for providing threshold voltage compensation in accordance with the

present invention;

Fig. 4 shows plots of characteristics for the complementary driver transistors

illustrated in Fig. 3 for various levels of threshold voltages;

Fig. 5 shows a driver circuit arranged to operate as a voltage driver circuit in

accordance with a first embodiment of the present invention.

Fig. 6 shows a driver circuit arranged to operate as a current programmed driver

circuit in accordance with a second embodiment of the present invention;

Fig. 7 shows a current programmed driver circuit in accordance with a third

embodiment of the present invention;

Figs 8 to 11 show SPICE simulation results for the circuit illustrated in Fig. 6;

Fig. 12 is a schematic sectional view of a physical implementation of an OEL element and driver according to an embodiment of the present invention;

Fig. 13 is a simplified plan view of an OEL elementOEL display panel incorporating the present invention;

Fig. 14 is a schematic view of a mobile personal computer incorporating a display device having a driver according to the present invention; .

Fig. 15 is a schematic view of a mobile telephone incorporating a display device having a driver according to the present invention,

Fig. 16 is a schematic view of a digital camera incorporating a display device having a driver according to the present invention,

Fig. 17 illustrates the application of the driver circuit of the present invention to a magnetic RAM, and Fig. 18 illustrates an alternative application of the driver circuit of the present invention to a magnetic RAM, and

Fig. 19 illustrates the application of the driver circuit of the present invention to a magnetoresistive element.

The concept of a driver circuit according to the present invention is illustrated in

Fig. 3. An OEL element is coupled between two transistors T₁₂ and T₁₅ which operate, in

combination, as an analog current control for the current flowing through the OEL element.

Transistor T₁₂ is a p-channel transistor and transistor T₁₅ is an n-channel transistor which act

therefore, in combination, as a complementary pair for analog control of the current through the OEL element.

As mentioned previously, one of the most important parameters in a TFT analog

circuit design is the threshold voltage N_τ. Any variation, ΔN_T within a circuit has a

significant effect on the overall circuit performance. Variations in the threshold voltage can

be viewed as a rigid horizontal shift of the source to drain current versus the gate to source

voltage characteristic for the transistor concerned and are caused by the interface charge at

the gate of the transistor.

It has been realised with the present invention that in an array of TFT devices, in

view of the fabrication techniques employed, neighbouring or relatively close TFT's have a

high probability of exhibiting the same or an almost similar value of threshold voltage ΔN_T.

Furthermore, it has been realised that as the effects of the same ΔN_T on p-channel and n-

channel TFT's are complementary, compensation for variations in threshold voltage ΔN_T

can be achieved by employing a pair of TFT's, one p-channel TFT and one n-channel TFT,

to provide analog control of the driving current flowing to the OEL element. The driving current can, therefore, be provided independently of any variation of the threshold voltage.

Such a concept is illustrated in figure 3.

Figure 4 illustrates the variation in drain current, that is the current flowing through

the OEL element shown in figure 3, for various levels of threshold voltage ΔN_T, ΔN_T1, ΔN^

for the transistors T₁₂ and T₁₅. Noltages V_l9 N₂ and N_D are respectively the voltages

appearing across transistor T₁₂, T₁₅ and the OEL element from a voltage source N_DD.

Assuming that the transistors T₁₂ and T₁₅ have the same threshold voltage and assuming that

ΔN_T = O, then the current flowing through the OEL element is given by cross-over point A

for the characteristics for the p-channel transistor T₁₂ and the n-channel transistor T₁₅ shown in figure 4. This is shown by value I₀.

Assuming now that the threshold voltage of the p-channel and n-channel transistors

changes to ΔN_T1, the OEL element current ϊ_λ is then determined by crossover point B.

Likewise, for a variation in threshold voltage to ΔN₂, the OEL element current I₂ is given

by crossover point C. It can be seen from figure 4 that even with the variations in the

threshold voltage there is minimal variation in the current flowing through the OEL

element.

Figure 5 shows a pixel driver circuit configured as a voltage driver circuit. The

circuit comprises p-chanήel transistor T₁₂ and n-channel transistor T₁₅ acting as a

complementary pair to provide, in combination, an analog current control for the OEL

element. The circuit includes respective storage capacitors C₁₂ and C₁₅ and respective

switching transistors T_A and T_B coupled to the gates of transistors T₁₂ and T₁₅. When

transistors T_A and T_B are switched ON data voltage signals N, and V₂ are stored respectively

in storage capacitors C₁₂ and Cj₅ when the pixel is not addressed. The transistors T_A and T_B function as pass gates under the selective control of addressing signals φ_x and φ₂ applied to

the gates of transistors T_A and T_B.

Figure 6 shows a driver circuit according to the present invention configured as a

current programmed OEL element driver circuit. As with the voltage driver circuit, p-

channel transistor T₁₂ and n-channel transistor T₁₅ are coupled so as to function as an analog

current control for the OEL element. Respective storage capacitors C,, and respective

switching transistors T_x and T₆ are provided for transistors T₁₂ and T₁₅. The driving

waveforms for the circuit are also shown in figure 6. Either transistors T_l5 T₃ and T₆, or

transistor T₄ can be ON at any one time. Transistors T_x and T₆ connect respectively

between the drain and gate of transistors T₁₂ and T₁₅ and switch in response to applied

waveform N_SEL to toggle transistors T_]2 and T₁₅ to act either as diodes or as transistors in

saturation mode. Transistor T₃ is also connected to receive waveform N_SEL. Transistors Tj

and T₆ are both p-channel transistors to ensure that the signals fed through these transistors

are at the same magnitude. This is to ensure that any spike currents through the OEL

element during transitions of the waveform N_SEL are kept to a minimum.

The circuit shown in figure 6 operates in a similar manner to known current

programmed pixel driver circuits in that a programming stage and a display stage are

provided in each display period but with the added benefit that the drive current to the OEL

element is controlled by the complementary opposite channel transistors T₁₂ and T₁₅.

Referring to the driving waveforms shown in figure 6, a display period for the driver circuit

extends from time to to time t₆. Initially, transistor T₄ is ON and transistors T_x, T₃ and T₆

are OFF. Transistor T₄ is turned OFF at time t_x by the waveform N_GP and transistors T_ls T₃

and T₆ are turned ON at time t₃ by the waveform N_SEL. With transistors T_x and T₆ turned

ON, the p-channel transistor T₁₂ and the complementary n-channel transistor T₁₅ act in a first mode as diodes. The driving waveform for the frame period concerned is available

from the current source I_DAT at time ^ and this is passed by the transistor T₃ when it

switches on at time t₃. The detected threshold voltages of transistors T₁₂ and T₁₅ are stored

in capacitors and C₂. These are shown as imaginary voltage sources ΔN_Tι₂ and ΔN_Tι₅ in

figure 6.

Transistors Υ T₃ and T₆ are then switched OFF at time t₄ and transistor T₄ is

switched ON at time t₅ and the current through the OEL element is then provided from the

source NDD under the control of the p-channel and n-channel transistors T₁₂ and T₁₅

operating in a second mode, i.e. as transistors in saturation mode. It will be appreciated

that as the current through the OEL element is controlled by the complementary p-channel

and n-channel transistors T₁₂ and Tι₅, any variation in threshold voltage in one of the

transistors will be compensated by the other opposite channel transistor, as described

previously with respect to figure 4.

In the current programmed driver circuit shown in figure 6, the switching transistor

T₃ is coupled to the p-channel transistor T₁₂, with the source of the driving waveform I_DAT

operating as a current source. However, the switching transistor T₃ may as an alternative

be coupled to the n-channel transistor T₁₅ as shown in figure 7, whereby I_DAT operates as a

current sink. In all other respects the operation of the circuit shown in figure 7 is the same

as for the circuit shown in figure 6.

Figures 8 to 11 show a SPICE simulation of an improved pixel driver circuit

according to the present invention.

Referring to figure 8, this shows the driving waveforms I_DAT, V_GP, N_SEL and three

values of threshold voltage, namely -lvolt, Ovolts and + lvolt used for the purposes of

simulation to show the compensating effect provided by the combination of the p-channel and n-channel transistors for controlling the current through the OEL element. From figure

8, it can be seen that, initially the threshold voltage ΔN_T was set at -lvolt, increasing to

Ovolts at 0.3 x IO^"4 seconds and increasing again to + lvolt at 0.6 x IO^"4 seconds. However,

it can be seen from figure 9 that even with such variations in the threshold voltage the

driving current through the OEL element remains relatively unchanged.

The relative stability in the driving current through the OEL element can be more

clearly seen in figure 10, which shows a magnified version of the response plots shown in

figure 9.

It can be seen from figure 10 that, using a value of 0 volts as a base for the threshold

voltage ΔN_T, that if the threshold voltage ΔN_T changes to -1 volts there is a change of

approximately 1.2% in the drive current through the OEL element and if the threshold

voltage ΔN_T is changed to + lvolt, there is a reduction in drive current of approximately

1.7% compared to the drive current when the threshold voltage ΔN_T is 0 volts. The

variation of drive current of 8.7% is shown for reference purposes only as such a variation

can be compensated by gamma correction, which is well known in this art and will not

therefore be described in relation to the present invention.

Figure 11 shows that for levels of I_DAT ranging from 0.2μA to l.OμA, the improved

control of the OEL element drive current is maintained by the use of the p-channel and

opposite n-channel transistors in accordance with the present invention.

It will be appreciated from the above description that the use of a p-channel

transistor and an opposite n-channel transistor to provide, in combination, analog control of

the drive current through an electroluminescent device provides improved compensation for the effects which would otherwise occur with variations in the threshold voltage of a single

p-channel or n-channel transistor.

Preferably, the TFT n-channel and p-channel transistors are fabricated as

neighbouring or adjacent transistors during the fabrication of an OEL elementOEL display

so as to maximise the probability of the complementary p-channel and n-channel transistors

having the same value of threshold voltage ΔN_T. The p-channel and n-channel transistors

may be further matched by comparison of their output characteristics.

Figure 12 is a schematic cross-sectional view of the physical implementation of the pixel driver circuit in an OEL element structure. In figure 12, numeral 132 indicates a hole injection layer, numeral 133 indicates an organic EL layer, and numeral 151 indicates a resist or separating structure. The switching thin-film transistor 121 and the n-channel type current- thin-film transistor 122 adopt the structure and the process ordinarily used for a low- temperature polysilicon thin-film transistor, such as are used for example in known thin-film transistor liquid crystal display devices such as a top-gate structure and a fabrication process wherein the maximum temperature is 600°C or less. However, other structures and processes are applicable.

The forward oriented organic EL display element 131 is formed by: the pixel electrode

115 formed of Al, the opposite electrode 116 formed of ITO, the hole injection layer 132, and the organic EL layer 133. In the forward oriented organic EL display element 131, the direction of current of the organic EL display device can be set from the opposite electrode

116 formed of ITO to the pixel electrode 115 formed of Al.

The hole injection layer 132 and the organic EL layer 133 may be formed using an ink-jet printing method, employing the resist 151 as a separating structure between the pixels. The opposite electrode 116 formed of ITO may be formed using a sputtering method. However, other methods may also be used for forming all of these components.

The typical layout of a full display panel employing the present invention is shown schematically in figure 13. The panel comprises an active matrix OEL element 200 with analogue current program pixels, an integrated TFT scanning driver 210 with level shifter, a flexible TAB tape 220, and an external analogue driver LSI 230 with an integrated RAM controller. Of course, this is only one example of the possible panel arrangements in which the present invention can be used.

The structure of the organic EL display device is not limited to the one described here. Other structures are also applicable.

The improved pixel driver circuit of the present invention may be used in display

devices incorporated in many types of equipment such as mobile displays e.g. mobile

phones, laptop personal computers, DND players, cameras, field equipment; portable

displays such as desktop computers, CCTN or photo albums; or industrial displays such as

control room equipment displays.

Several electronic apparatuses using the above organic electroluminescent display device will now be described. <1: Mobile Computer>

An example in which the display device according to one of the above embodiments is applied to a mobile personal computer will now be described.

Figure 14 is an isometric view illustrating the configuration of this personal computer. In the drawing, the personal computer 1100 is provided with a body 1104 including a keyboard 1102 and a display unit 1106. The display unit 1106 is implemented using a display panel fabricated according to the present invention, as described above. <2: Portable Phone>

Next, an example in which the display device is applied to a display section of a portable phone will be described. Fig. 15 is an isometric view illustrating the configuration of the portable phone. In the drawing, the portable phone 1200 is provided with a plurality of operation keys 1202, an earpiece 1204, a mouthpiece 1206, and a display panel 100. This display panel 100 is implemented using a display panel fabricated according to the present invention, as described above. <3: Digital Still Camera>

Next, a digital still camera using an OEL display device as a finder will be described. Fig. 16 is an isometric view illustrating the configuration of the digital still camera and the connection to external devices in brief.

Typical cameras sensitize films based on optical images from objects, v/hereas the digital still camera 1300 generates imaging signals from the optical image of an object by photoelectric conversion using, for example, a charge coupled device (CCD). The digital still camera 1300 is provided with an OEL element 100 at the back face of a case 1302 to perform display based on the imaging signals from the CCD. Thus, the display panel 100 functions as a finder for displaying the object. A photo acceptance unit 1304 including optical lenses and the CCD is provided at the front side (behind in the drawing) of the case 1302.

When a cameraman determines the object image displayed in the OEL element panel 100 and releases the shutter, the image signals from the CCD are transmitted and stored to memories in a circuit board 1308. In the digital still camera 1300, video signal output terminals 1312 and input/output terminals 1314 for data communication are provided on a side of the case 1302. As shown in the drawing, a television monitor 1430 and a personal computer 1440 are connected to the video signal terminals 1312 and the input/output terminals 1314, respectively, if necessary. The imaging signals stored in the memories of the circuit board 1308 are output to the television monitor 1430 and the personal computer 1440, by a given operation.

Examples of electronic apparatuses, other than the personal computer shown in Fig. 14, the portable phone shown in Fig. 15, and the digital still camera shown in Fig. 16, include OEL element television sets, view-finder-type and momtoring-type video tape recorders, car navigation systems, pagers, electronic notebooks, portable calculators, word processors, workstations, TN telephones, point-of-sales system (POS) terminals, and devices provided with touch panels. Of course, the above OEL device can be applied to display sections of these electronic apparatuses.

The driver circuit of the present invention can be disposed not only in a pixel of a display unit but also in a driver disposed outside a display unit.

In the above, the driver circuit of the present invention has been described with reference to various display devices. The applications of the driver circuit of the present invention are much broader than just display devices and include, for example, its use with a magnetoresistive RAM, a capacitance sensor, a charge sensor, a DΝA sensor, a night vision camera and many other devices.

- Figure 17 illustrates the application of the driver circuit of the present invention to a magnetic RAM. In figure 17 a magnetic head is indicated by the reference MH.

Figure 18 illustrates an alternative application of the driver circuit of the present invention to a magnetic RAM. In figure 18 a magnetic head is indicated by the reference MH.

Figure 19 illustrates the application of the driver circuit of the present invention to a magnetoresistive element. In figure 19 a magnetic head is indicated by the reference MH. and a magnetic resistor is indicated by the reference MR. The aforegoing description has been given by way of example only and it will be

appreciated by a person skilled in the art that modifications can be made without departing

from the scope of the present invention.

Claims

1. A driver circuit for a current driven element, the circuit comprising an n-channel

transistor and a complementary p-channel transistor connected so as to operatively control,

in combination, the current supplied to the current driven element.

2. A driver circuit as claimed in claim 1, wherein the complementary n-channel and p-

channel transistors comprise polysilicon thin film transistors.

3. A driver circuit as claimed in claim 2, wherein the complementary n-channel and p-

chahnel transistors are spatially arranged in close proximity to each other for providing a

complementary pair of n-channel and p-channel transistors having approximately equal

threshold voltages.

4. A driver circuit as claimed in any one of claims 1 to 3 connected so as to establish

when operative a voltage driver circuit comprising respective storage capacitors for the n-

respective data voltage pulses.

5. A driver circuit as claimed in any one of claims 1 to 3 comprising respective storage

capacitors for storing a respective operating voltage of the n-channel and the p-channel

transistors during a programming stage, a first switching means connected so as to establish when operative a first current path from a source of current data signals through the n-

channel and p-channel transistors and the current driven element during the programming

stage, and a second switching means connected to establish when operative a second current

path through the n-channel and p-channel transistors and the current driven element during a

reproduction stage.

6. A driver circuit as claimed in claim 5, wherein the first switching means and the

source of current data signals are connected so as to provide when operative a current source for the current driven element.

7. A driver circuit as claimed in claim 5, wherein the first switching means and the

source of current data signals are connected so as to provide when operative a current sink

for the current driven element.

8. A driver circuit as claimed in any one of claims 5 to 7, further comprising respective

further switching means respectively connected to bias the n-channel transistor and the p-

channel transistor to act as diodes during the prograrnming stage.

9. A driver circuit as claimed in claim 8, wherein the respective further switching

means comprise p-channel transistors.

10. A driver circuit as claimed in any one of claims 5 to 9, wherein the circuit is

implemented with polysilicon thin film transistors.

11. A driver circuit as claimed in claim 4, wherein the circuit is implemented using

polysilicon thin film transistors.

12. A driver circuit as claimed in any preceding claim, wherein the current driven

element is an electroluminescent element.

13. A method of controlling the supply current to a current driven element comprising

providing an n-channel transistor and a p-channel transistor connected so as to operatively

control, in combination, the supply current to the current driven element.

14. A method as claimed in claim 13, comprising the further step of providing the n-

channel transistor and the p-channel transistor as polysilicon thin film transistors.

15. A method as claimed in claim 14 comprising the further step of spatially arranging

the n-channel and p-channel polysilicon thin film transistors in close proximity to each

other.

16. A method as claimed in any one of claims 13 to 15 comprising providing respective

storage capacitors for the n-channel and p-channel transistors and respective switching

means connected so as to establish when operative respective paths to the n-channel and p-

channel transistors for respective data voltage pulses thereby to establish, when operative, a

voltage driver circuit for the current driven element.

17. A method as claimed in any one of claims 13 to 15 comprising providing a

programming stage during which the n-channel and p-channel transistors are operated in a

first mode and wherein a current path from a source of current data signals is established

through the n-channel and the p-channel transistors and the current driven element and

wherein a respective operating voltage of the n-channel transistor and the p-channel

transistor is stored in respective storage capacitors, and a reproduction stage wherein a

second mode and a second current path is established through the n-channel transistor and

the p-channel transistor and the current driven element.

18. A method as claimed in claim 17, wherein the first mode comprises operating the n-

channel and p-channel transistors as diodes.

19. A method of controlling the supply current to an electroluminescent display

comprising the method as claimed in any one of claims 13 to 18 wherein the current driven

element is an electroluminescent element.

20. An organic electroluminescent display device comprising a driver circuit as claimed

in any one of claims 1 to 12. . ^{■ *} .

21. An electronic apparatus incorporating an organic electroluminescent display device as claimed in claim 20.

22. A circuit comprising a current driven element and at least two active elements, the

current driven element being disposed between the two active elements.

23. A circuit comprising a current driven element and at least two active elements, the

two active elements being connected through the current driven element together.

24. The circuit according to claim 22 or claim 23, wherein the two active elements are

transistors.

25. The circuit according to claim 24, wherein the two transistors are mutually different

channel type transistors.

26. the circuit according to claim 22 or claim 23, wherein the current driven element is

an organic elecfroluminescent element.

27. The circuit according to claim 24, wherein the gates of the two transistors are each

connected to a respective capacitor.

28. An electro-optical device comprising the circuit according to claim 22.

29. An electronic apparatus incorporating an electro-optical device according to claim

28.

30. A method for driving a circuit comprising a current driven element, a first active

element, and a second active element that is disposed at a side of the current driven element opposite to the first active element, controlling a current supplied to the current driven

element by the first active element and the second active element.

31. The method according to claim 30, comprising the step of selecting the first active

element to be a first transistor and selecting the second active element to be a second

transistor.

32. The method according to claim 31, comprising a step of determining a gate voltage

of at least one of the first transistor and the second transistor based on a predetermined

current.

33. The method according to claim 32, comprising the step of causing the predetermined

current to flow through a second current path different from a first current path that

includes the current driven element.

34. The method according to claim 33, comprising the step of arranging the second

current path to include at least one of the first transistor and the second transistor.